On the combustion of heavy oil, the mass of discharge of air pollutive substances (such as nitrogen oxides, sulfur oxides, carbon dioxide, dust, smog etc.) cannot be avoided, so the research and development of combustion technology preventing the production of such substances has been conducted for a long time. It is indispensable in order to burn heavy oil having high viscosity effectively that heavy oil is heated before the spray at burner and is brought to lower viscosity so that the oil is possible to be sprayed as small particle, and that its evaporation speed and followed mixing with air (oxygen) is accelerated for the diffusion combustion. But nevertheless, it has not brought the problem to solution and the development on technology for the burner and the treatment of combustion gas etc. is still being continued. But there is big burden on the site of the users because of facility cost, installation area, maintenance of plant etc. and therefore efficient combustion based on simplified technology has been required.
One of combustion technologies which was developed in consideration of above problems is an oil-water emulsion fuel in which oil is mixed with water. This oil-water emulsion fuel has an improved combustion efficiency because water particles micro-explode due to the discrepancy of the boiling points between heavy oil and water (b.p. of heavy oil is 300.degree. C. or more, b.p. of water is 100.degree. C.) so that the explosion divides the oil into finer particles and it leads to promotion of diffusion combustion when the emulsion fuel is sprayed into combustion chamber having high temperature. And with this fuel, the production of nitrogen oxides can be decreased owing to the combustion at high temperature. These are the main object for which oil-water emulsion fuel has been developed. The oil-water emulsion fuel is prepared by two processes for production roughly classified, one of which is the method where only oil and water are mechanically mixed and the other one of which is the method where oil and water are mixed with chemical additives (organics).
While the conventional oil-water emulsion fuels have attained the effect that they decrease the production of some of air pollutive substances (nitrogen oxides) by a few ppm, they have not attained good result in combustion efficiency. They still have the problem as follows:
1) In the heating process for lowering the viscosity of heavy oil having high viscosity, the cohesion happens among water particles and the oil and water separate. Therefore, the spraying with maintaining constant particle size of water, which is a essential factor for ideal micro-explosion, cannot be conducted at all.
2) The measurement of viscosity and total calorific power and analysis of components from the change of temperature are difficult as the oil-water separation happens by heating.
3) There is a problem in maintenance that fuel pipes and burners etc. are oxidized by water during the combustion.
4) The concentration of nitrogen oxides and smoke increase more in the combustion at lower oxygen atmosphere than in the combustion of heavy oil only and the condition of the combustion apparently becomes worse. There is a possibility of the production of new pollutive substances in the case of using chemical additives because they are mainly consist of organic components.
5) The transparency of the inside of combustion chamber is very bad and the flame grows thin and long and the state where the oil particles are running can be observed by eyes. This means the combustion is too far from good condition.
6) When the combustion chamber and flue etc. is checked after combustion, it is observed that the thick deposit of impurities is observed on the inside of them and the quantity is high.